[this is probably not the answer you want to read]

Because Ticketmaster has been a force in the market for decades (at least since the 1980’s), the simplest market based explanation is that using Ticketmaster is often obviously the economically rational choice.

For example, many people who dislike Ticketmaster choose to buy tickets through Ticketmaster rather than exercise their alternatives. The same is true for performers and venues.

Because that is how markets work.

Any potential competitor has to do some, many. or all the things Ticketmaster does…not the least of which is staying in business…and that’s non-trivial.

Or at least that is what ordinary market economic theory strongly suggests.